1. Field of the Invention
The present invention relates to color processing for mapping image data on an output color gamut.
2. Description of the Related Art
Along with the popularization of personal computers, many people use image input devices such as digital cameras and image scanners. Therefore, opportunities to input an image by an image input device, to display that image on an image display device such as a CRT, LCD, or the like, and to output the image using an image output device such as a printer or the like are increasing. In this case, color matching processing (gamut mapping) is required to correct image perception differences due to different color gamuts among the image input, display, and output devices. That is, the color matching processing absorbs color perception differences among devices.
As one method of gamut mapping, so-called calorimetric processing is known. The calorimetric processing expresses a color gamut common to an input device and output device intact. As for a non-common color gamut, the processing maps colors of the input device on a boundary surface of a color gamut of the output device (to be referred to as “output color gamut” hereinafter).
Various techniques of mapping methods have been proposed. For example, a method of checking if an input color falls within an output color gamut, and mapping a color outside the output color gamut at a nearest neighboring point of the boundary surface of the output color gamut, a method of mapping a color outside the output color gamut at a point having the same lightness and hue values and a maximum chroma value, and the like are available. With the former method, the hue of the color outside the output color gamut changes, and one may feel discrepancy in the color of an output image. With the latter method, the lightness and hue are maintained, but the chroma lowers considerably depending on the compressed color gamut of the input device (to be referred to as “input color gamut” hereinafter), and one may feel an unnatural image. Furthermore, both these methods completely ignore the tonality of colors outside the output color gamut.
To solve such problem, for example, as described in E. G. Pariser, “An Investigation of Color Gamut Reduction Techniques”, IS & T Symp. Elec. Prepress Tech.-Color Printing, pp. 105-107 (1991), a mapping method based on a focal point scheme that executes gamut mapping toward one point (50, 0, 0) on the gray axis on the CIELAB space has been proposed. Also, Japanese Patent Laid-Open No. 8-046776 (U.S. Pat. No. 5,745,263) describes a method of adaptively setting a focal point (mapping convergent point). An upper limit and lower limit are set on the lightness axis of the output color gamut. When the lightness of an input point is larger than the upper limit, the upper limit is set as a focal point. On the other hand, when the lightness of an input point is lower than the lower limit, the lower limit is set as a focal point. When the lightness of an input point falls between the upper limit and lower limit, a point on the lightness axis having the same lightness as the input point is set as a focal point.
Many mapping methods in the calorimetric processing have been proposed. However, since the above two methods preserve hues of out-of-gamut colors and express tones albeit slightly, an output image which is less unnatural is obtained. However, since the mapping method based on the focal point scheme uniquely determines a mapping direction, it cannot cope with a mapping mode that the user requires and it cannot perform proper mapping when the output color gamut is changed. For example, assume that the user requires chroma-oriented mapping that reduces a chroma difference before and after the mapping or lightness-oriented mapping that reduces a lightness difference before and after the mapping. In this case, the above two methods cannot switch the focal point, and cannot meet the user's requirement.
The output color gamut of an output device often largely changes depending on the combinations of media and inks. For example, for a medium such as glossy paper that can output high-chroma colors, a chroma difference before and after the mapping is not so large even when mapping is made while maintaining the lightness of an input color. Therefore, it is desirable to set the focal point for lightness-oriented mapping. On the other hand, in case of a medium such as plain paper that can only output low-chroma colors, a chroma difference before and after the mapping becomes very large if mapping is made while maintaining the lightness of an input color. Therefore, a better image can be obtained by setting the focal point for mapping that attaches importance to chroma rather than lightness. The above two methods cannot cope with switching of mapping methods corresponding to such change in output color gamut.